Long-term nitrogen deposition does not affect nitrous oxide, methane and carbon dioxide exchange of mature beech tree stems

European beech (Fagus sylvatica L.) is a native and widely grown tree species typical for upland forests of Central and Southeast Europe. The soils of beech forests are regarded as predominant sources of nitrous oxide (N₂O), sinks of methane (CH₄) and sources of carbon dioxide (CO₂), while the contribution of beech trees themselves to the ecosystem greenhouse gas (GHG) exchange varies by gas species and site. Moreover, forest nitrogen (N) and carbon cycling, and thus N₂O, CH₄ and CO₂ turnover processes are affected by N deposition, yet, the long-term effect of N deposition on GHG exchange of soil and mature trees is far from being understood.

We aimed to investigate whether an increase in N deposition can alter forest GHG emissions. In September 2023, we measured N₂O, CH₄ and CO₂ exchange of beech stems and adjacent soil, and various environmental parameters in a mature pre-alpine beech forest in Northeastern Italy, where a N manipulation experiment (4 treatments each replicated in 3 plots) has been carried out since 2015. Four experimental plots were selected: control (N0, only ambient deposition), canopy N addition (N30A, +30 kg ha^-1 yr^-1 sprayed over tree canopies) and soil N additions with two different doses (N30 and N60, +30 and +60 kg ha^-1 yr^-1, respectively).

The stems of mature beech trees were net sinks of CH₄ (-11.9 ± 3.6 mg ha^-1 ground area h^-1, median ± 95% confidence interval) and sources of CO₂ (639 ± 137 g ha^-1 h^-1), their N₂O exchange potential (3.36 ± 3.82 mg ha^-1 h^-1) was rather low. The stem fluxes of all three GHGs were not affected by nine years of N treatment.

The long-term N deposition did not alter the soil CO₂ emission (3015 ± 193 g ha^-1 h^-1). However, the N addition to the soil tended to increase the soil CH₄ uptake (-642 ± 61 versus -901 ± 69 mg ha^-1 h^-1, N0+N30A versus N30+N60 plots). The soil N₂O emissions were highest at the control plot (82.4 ± 33.9 mg ha^-1 h^-1), whereas the plots N30A and N60 showed significantly lower fluxes (1.56 ± 12.41 mg ha^-1 h^-1).

Our preliminary results detected high spatial variability in stem and soil GHG fluxes, which might be rather connected to the variable site topography than to the long-term N deposition effect. Future detailed soil GHG flux measurements across all experimental plots replicates will help to understand this variability and the effect of N deposition on the GHG fluxes.

 

Acknowledgement

This research was supported by the Ministry of Education, Youth and Sports of CR within the programs CzeCOS (grant number LM2023048) and LU - INTER-EXCELLENCE II (grant number LUC23162). We thank Federico Magnani and Alessandra Teglia from the University of Bologna and Reparto Carabinieri Biodiversità in Pian del Cansiglio for scientific and logistic support, respectively.